L-Amino acids are used as ingredients in medicines for humans and are applied particularly to the pharmaceutical industry and the food industry and are also used as nutrients for animals. Among L-amino acids, L-ornithine, one of the products on the arginine cycle, is known as a medicinal component that enhances liver function (Salvatore et al., 1964). L-Arginine is abundantly found as a free form in plant seeds and garlic and is used as a nutritional supplement in medicinal or food products. Examples of the medicinal applications of L-arginine include anti-hepatotoxic substances, cerebral metabolic enhancers, therapeutics for male infertility, and general amino acid formulations. Representative among the foods to which L-arginine is applied are fish paste additives, health beverage additives and salt alternatives for patients with hypertension.
Fermentation is applied when coryneform bacteria (especially Corynebacterium glutamicum) is used in the mass production of amino acids. Due to the high significance thereof in industrial terms, the method of bacterial production of amino acids is continuously being improved upon. Methodological improvements have been achieved with respect to, for example, agitation, oxygen introduction, and the maintenance of sugar concentration during fermentation.
To increase the microbial productivity of amino acids, the selection of suitable microbes is very important L-amino acid-producing strains which are resistant to anti-metabolite substances or auxotrophic for metabolites responsible for the regulation of amino acids may be selected. For example, variants of Brevibacterium or Corynebacterium spp. which produce glutamate are used to produce ornithine (EP 0 393 708 A3). Also, variants of glutamate-producing Brevibacterium or Corynebacterium spp. are used to directly produce L-arginine from carbon and nitrogen sources (Japanese Patent Publication NOs. Sho. 57-163487, 60-83593 and 62-265988).
Recombinant DNA technology is also a useful tool by which L-amino acid-producing coryneform strains can be genetically altered to enhance functions associated with the production of L-amino acids. For example, argCJBD, an ornithine biosynthesis gene, may be introduced into and overexpressed in a strain which is incapable of synthesizing arginine and proline (Hwang et al., 2008). Also, a strain may be genetically recombined to inactivate argR, a gene repressing the expression of the arginine biosynthesis operon (U.S. Patent Application Publication No. 2002/0045223A1).
An increase in amino acid productivity can be achieved by reinforcing the biosynthesis of genes of interest. A report has it that the yield of amino acid production can be improved upon by increasing the expression of biosynthesis genes. For example, the overexpression of the arginine biosynthesis gene argF leads to an increased production of arginine (Korean Patent Application No. 10-2004-107215).
In addition, a method for producing L-arginine in which the argD2 gene (Ncgl2355) or (Ncgl0990), a putative gene of acetylornithine aminotrasferase involved in the arginine biosynthesis of Corynebacterium glutamicum, is overexpressed to produce L-arginine in high yield is disclosed (Korean Patent Nos. 0830289 and 0830290).
Like this, microbial strains capable of producing ornithine or arginine in high yield may result from the reinforcement of biosynthesis genes, especially genes of the biosynthesis enzymes acetylglutamate synthase and acetylornithinase.
However, acetylglutamate synthase has been found in none of the Corynebacterium spp. known thus far. In this invention, the present invention examined a gene which codes for an enzyme responsible for the function of acetylglutamate synthase. It is reported that the polypeptide of the gene has an activity similar to that of acetylornithinase, encoded by argJ. Further, an enhancement in the activity of the gene was found to increase the concentration of ornithine or arginine. Based on these research results, the present invention was accomplished.